This application claims the priority of German patent document 198 16 978.7, filed Apr. 17, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for identifying an incorrectly measuring sensor which is part of a sensor arrangement in a spacecraft for measuring directional vectors.
Spacecraft, particularly earth satellites which must maintain a predetermined orientation on their orbit with respect, for example, to a defined orbital system of coordinates, are equipped with attitude control systems. The latter must have sensors whose measurements provide three-axis attitude information. In the case of earth satellites, these may include for example, earth sensors, sun sensors, star sensors or magnetometers. Earth, sun and star sensors measure directional vectors, which indicate the direction of external objects (specifically the earth, the sun or selected stars) with respect to the actual location of the spacecraft. In the case of an earth satellite, a magnetometer measures the intensity and the direction of the earth's magnetic field at the actual location of the satellite, and a corresponding directional vector is also obtained.
All such directional vectors may be expressed as unit vectors relative to a spacecraft-fixed system of coordinates, which may consist, for example, of three orthogonal axes X, Y, Z. In the case of an earth satellite, these axes are usually aligned with the Z-axis (yaw axis) pointing to the center of the earth; the X-axis (roll axis) pointing in the orbiting direction; and the Y-axis (pitch axis) situated perpendicularly on the two other axes or the orbiting plane of the spacecraft.
From the measured directional vectors, conclusions can be drawn concerning any incorrect orientation of the spacecraft about one or several of the three axes of the spacecraft-fixed system of coordinates, if the vectors do not correspond to definable reference vectors. The orientation of the spacecraft can then be corrected by means of corresponding control elements which generate controlling torques, such as reaction nozzles, swirl wheels or magnetic torque generators.
In attitude control systems of this type, it is very important that errors in the measurement of the attitude or of the direction vectors be recognized automatically in order to prevent major negative effects of such a sensor error in time. Known error recognition measures consist of a plausibility examination of the measured values of the sensor. Plausibility criteria include for example, whether a measurement remains absolutely constant over a longer period of time, whether it results only in values within a certain range, or whether the change of a measurement exceeds a certain extent. Such monitoring frequently requires high programming expenditures if, for example, an angle measurement consists of many partial measurements, each of which can be incorrect. Furthermore, they are difficult to implement because the pertaining error limits cannot easily be indicated. If the latter are too stringent, they are exceeded too easily without the actual presence of an error. However, if they are too lax, an error may not be recognized in time.
It is therefore an object of the present invention to provide a process by means of which incorrectly measuring sensors can be recognized more effectively.
This and other objects and advantages are achieved by the process according to the invention, in which the respective actual external directional vectors are first computed with respect to an inertial system of coordinates, using information concerning the actual point in time as well as the actual location of the spacecraft. Alternatively, the mentioned information can be received via an earth station, determined by analyzing GPS data on board the spacecraft, or determined by means of a time-dependent model of the orbital course of the spacecraft stored on board the spacecraft. For example, in the case of an earth satellite, such an inertial system of coordinates can be selected in which one axis is aligned perpendicularly with respect to the earth orbit plane; the second axis is aligned in parallel to the direction of the sun center-spring point of the earth orbit; and the third axis is aligned perpendicularly to the two above-mentioned axes. This system of coordinates may be conceived to be centered in the center of the earth and orbit along with the earth on its path around the sun. According to the date and the time of day, and with the additional knowledge of the location of the earth satellite on its orbit, the directional vectors which are of interest can then be calculated: for example, the earth vector which is directed from the location of the satellite to the center of the earth; the sun vector directed toward the center of the sun; one or several star vectors directed to preselectable stars; as well as finally the magnetic field vector which exists in each case at the satellite location. The thus calculated directional vectors are related to the inertial system of coordinates.
According to the invention, the angles are then determined between the above-mentioned directional vectors, as well as the angles between directional vectors measured directly by the sensors. In the first-mentioned case, these angles are therefore determined between directional vectors which are defined in the inertial system of coordinates; and in the second case, between such directional vectors which are defined in the satellite-fixed or spacecraft-fixed system of coordinates. However, the angles themselves are independent of the system of coordinates in which they were each calculated. That is, if measured and calculated correctly, the corresponding angles (for example, the angle between the earth and sun vectors calculated in the inertial system of coordinates and the same angle measured in the spacecraft-fixed system of coordinates) must be identical.
Accordingly, the angles which correspond to one another with respect to the participating directional vectors (for example, the two above-mentioned angles which extend in each case between for the earth and sun vectors), from the two systems of coordinates are compared with one another. This comparison between the angles is independent of the actual alignment of the spacecraft or of the system of coordinates on which the directional vectors are based.
A sensor will be identified as measuring incorrectly if only in this comparison of angles all of those angles which are determined based on the directional vector measured by this sensor deviate from one another by more than a definable minimum amount.
The GPS data received by GPS satellites also permit a determination of the actual attitude of the spacecraft. From the latter, as well as from the information concerning the location and point in time (also received), the directional vectors can then be calculated with respect to the spacecraft-fixed system of coordinates. As a result, it is possible to make a comparison between these calculated directional vectors and the directional vectors measured on board the satellite by means of the sensors. The result of this comparison can then also be used to decide whether a sensor is measuring incorrectly.